1. Field of the Invention
This invention relates to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation, and photoelectrically detecting the emitted light. This invention particularly relates to a radiation image readout apparatus wherein a long or elongated photomultiplier is utilized as a means for detecting the light emitted by the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Patent Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved image quality, which allows the visible image to be used in making efficient and accurate diagnoses of illnesses.
In general, radiation image read-out apparatuses used in the aforesaid radiation image recording and reproducing systems are constituted by a main scanning means for scanning a stimulable phosphor sheet, on which a radiation image has been stored, with stimulating rays in a main scanning direction, i.e., along a main scanning line, a sub scanning means for moving the stimulable phosphor sheet with respect to the stimulating rays in a sub scanning direction approximately normal to the main scanning direction, and a photo detecting means for detecting light emitted by the stimulable phosphor sheet in proportion to the amount of energy stored during exposure to radiation.
Recently, a novel photo detecting means which utilizes a long photomultiplier was proposed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-16666. The disclosed long photomultiplier is provided with a cylindrical main body having a light receiving face which extends along the main scanning line on the stimulable phosphor sheet. A photocathode is provided on an inner surface of the main body along the light receiving face. Also, in general, a light guide member is located so that it is in close contact with the light receiving face and so that it extends along the light receiving face and guides the light emitted by the stimulable phosphor sheet toward the photocathode. With the long photomultiplier, non-directional light emitted by the stimulable phosphor sheet is guided by the light guide member toward the photocathode. When exposed to light emitted by the stimulable phosphor sheet, the photocathode generates photoelectrons, which are sequentially multiplied by the secondary electron emission effects of dynodes.
With the aforesaid long photomultiplier, light emitted by every portion of the stimulable phosphor sheet in the main scanning direction can be detected efficiently. Also, radiation image read-out apparatuses using the long photomultiplier can be made smaller than apparatuses using a photomultiplier in which a light guide member having a complicated shape is located so that it is in close contact with a small light receiving face, as disclosed in, for example, Japanese Unexamined Patent Publication No. 54(1979)-87808.
However, the aforesaid long photomultiplier has a drawback in that a large number of photoelectrons cannot readily be emanated from the photocathode. Specifically, in order to generate a large number of photoelectrons in the photocathode, it is necessary to increase the light absorption efficiency of the photocathode. For this purpose, the photocathode should be made thicker. However, photoelectrons generated at positions deep within the width of the photocathode cannot readily be emanated out of the photocathode. From this viewpoint, the photocathode should be made thinner. These two incompatible requirements make it difficult for a large number of photoelectrons to emanate from the photocathode.